Three years ago, Rufus Sewell gave up bad guys for good. After an early career steaming up teatimes in Middlemarch and Cold Comfort Farm, he slipped, in his 30s, into a rut of rotters – cruel toffs on horseback in films such as A Knight's Tale, The Legend of Zorro and Tristan and Isolde. At 40, he renounced them for better men. Men with names like Tom Builder (from HBO's The Pillars of the Earth) and Dr Jacob Hood (police procedural Eleventh Hour), and Zen, the cool Italian detective from the Michael Dibdin books adapted for the BBC.

Next week Sewell, now 44, can be seen as "Adam, lead vampire" in Abraham Lincoln: Vampire Hunter, a 3D film from Timur Bekmambetov, a movie about the 16th president discovering that vampires are planning to take over the world. Adam is 5,000 years old, rides a horse and tries to drink the blood of America's most beloved president. A return to the dark side? "Well, thing is," says Sewell, "after they cancelled Zen, I didn't work for eight months. And in that case, it was not my choice. After I've done something that I'm really proud of and I think changes the way I'm perceived, the immediate reaction is: nothing. After Charles II on TV, Rock'n'Roll on stage, and then Zen, I had the longest spells of unemployment. So by the time this offer came, I was really happy to play a bad guy. And it had really good people doing it. And I've never played a vampire before. So here I am."

We are meeting in Santa Monica, in the spring, when the film is not yet finished and all either of us has seen is an eight-minute showreel. "What I saw looked quite good," says Sewell. "Bits of it I didn't expect to look exciting looked exciting. Making it was fun but bewildering and I didn't know if that bewilderment would transfer to my performance. I'll stick my neck out and say I hope it's quite good. I'll go so far." Three months later, and that is still all that has been screened to the press, so it's hard to know if he's right.

The film was adapted from Seth Grahame-Smith's novel Abraham Lincoln: Vampire Hunter, but scour that in search of a curly-haired, sneery old vampire and you'll be disappointed. "Yes, he's not in the book. They had this idea to write this sort of uber-villain. Which is not the most organic way of coming up with a character. But so what?"

Sewell says all this not with a laugh but, rather, while laughing. He barely goes two words without cocking a snook at himself – his history, his hypocrisy, his hopeless attempts to research the role. "What accent should he have? Aramaic? No - people will just think you're a fucking idiot. Or you've got something stuck in you're throat. And they'll cut the scene when you mention you're 5,000." He was dismayed by a few frames that showed Adam in all his corpsey glory: "I was going to send a picture to my son, and then I was like: oooh, fuck that. I'd been thinking maybe I'd look really sexy. No, I look disgusting."

This does not appear to be an invitation to disagree. For an actor, especially, Sewell has unusually clear-eyed self-perception. Twenty years of not quite making it have borne in him not bitterness but a sort of jovial brutalism, bordering on scorn. Two years ago, promoting Zen, he was merrily sceptical about its chances, if not its merits. Today, he's even more off-message. He rails gleefully against Abraham Lincoln's "retrofitted" political subtext. "I've been asked to address all these things I think are bollocks. 'Are you a vampire or a patriot?' Oh, fuck off! I didn't realise that was the choice. I'm not keen on these talking points they hand out: this is what we think. Oh, is it? So I get a bit punchy. 'Why should people come and see your movie?' Don't! I don't care. I don't think that way, but it brings it out in me, and I feel a bit rubbish afterwards. I'm just not very comfortable toeing the line."

But at the same time, Sewell gamely bats back whatever question you throw at him in lieu of the movie. Yes, he thinks vampires are more attractive than other undead ("zombies aren't sexy, and werewolves are only sexy until the moon, then they're just hairy"). No, playing one did not lead him to reassess eating red meat. Yes, Americans are more eager to mythologise their leaders than Brits.

"One of the things I was proudest of when I was growing up was that we're not proud of our country. Anyone with any sense knows that we're mongrels and that it's been different family businesses topping off other different family businesses for years. There's no one we would care about trashing. Winston Churchill: Vampire Hunter would work as pure camp because it's not a risk. It's just: yeah, that's funny. No one's gonna be: [hushed voice] 'Could we?'"

Sewell has settled in the States now, rents a flat with "a girl who is not an actress", but still shuttles back and forth to London to see his son, Billy, 10. Yes, he says warily, LA life makes him feel more British. "But there's a certain type of ex-pat: 'Oh, thank God! Someone who gets my irony.' Oh, fuck off! Actually, some of the Americans are really droll. In terms of wanker per square mile, Notting Hill can still hold its own."

This he follows with a swift mea culpa: he can rival the worst of them in the insufferability stakes. After a half-bohemian, not-as-posh- as-you'd-think childhood in Twickenham, Sewell went off the rails after the death of his father when he was 10. A spell of being what he has called an "80s twat" – which translates as dyeing his hair, drinking, taking drugs, shoplifting and playing truant - gave way to standard-issue Soho louchery. When he first hopped the pond, he behaved in a way "that was slightly embarrassing to myself and I blame America for it. I'd just be in bars with strangers and after a while you feel a bit of a wanker. I was frightened of what I might become." What was that?

"Cheesy. Everyone would be telling you you're amazing all the time. It's scary because you get less embarrassed about it. Some people here talk about themselves like it's their one opportunity to make the sell. Even their imagined syndromes become part of their CV: hey, I'm ADHD and I'm partly bipolar and I'm a Sagittarius."

He often looks "confused" in his own movies, he thinks; a result of funking out of asking enough questions on set. "We're English. It's part of our makeup. If someone asks me about two books and I say I haven't read them; the third book - I'll say I've read it. You get embarrassed. And if someone explains an idea, twice, the second time with a sigh, and there's a crew waiting, it's very tempting to go: OK!"

Things will probably get easier as he ages, he grins. He tried to write a script himself, to showcase his own versatility, only to start resenting his own knee-jerk typecasting. "How dare you! I could do so much more!" For now, though, he's cheerily resigned to a fresh batch of cads; a happy self-flagellator, tickled by his own hubris. "My problem is I'm a fussy beggar. I'm starving! But white bread? Oh no ..."

•This article was amended on Friday 15 June, after we mis-spelled the name of film director Timur Bekmambetov as Tibor Bekmambetov. This has now been corrected.